Web printing systems are known that implement either a single reflex or a double reflex registration system to time the delivery of firing signals to printheads in a print zone of the web printing system. “Double reflex registration system” refers to a system that uses the angular velocity signals corresponding to the rotation of two or more rollers to compute the web velocity at a printhead positioned between the rollers. A single reflex registration system refers to a system that uses the angular velocity signals corresponding to the rotation of only one roller to compute a linear web velocity that is used to predict web positions and timing in a print zone. “Reflex mode” as used in this document refers to a printer being operated with either a double reflex registration system or a single reflex registration system. Some printers are capable of being operated in either reflex mode.
To implement reflex printing control, a controller can use tension measurements from load cells that provide measurements of web tension along with the angular velocity measurements from encoders associated with rollers in the web printing system to compute linear web velocities at various locations in the system. These linear velocities enable the processor to determine when a web portion previously printed by one marking station is opposite another marking station so the second marking station can be operated by the controller with firing signals to eject ink of a different color onto the web in proper registration with the ink already placed on the web by a previous marking station. Additionally, reflex printing controls enables the printing at any given marking station to be deposited in an equally spaced and uniform manner. Operation of a subsequent marking station either before or after the ejected ink arrives at the subsequent marking station may produce visual noise in the image. This effect is known as registration error. Accurate measurements, therefore, are important in registration of different colored images on the web to produce images with little or no visual noise. That is, accurate angular velocity measurements simplify the process of determining the linear velocity of the web at a particular position and the timing of the firing signals correlated to the linear web velocity.
Accurate measurements can be affected by changes arising from environmental conditions. For example, temperature changes may change the diameter of one or more roller cylinders significantly enough to affect the accuracy of the velocity computed for the web and the timing of the firing signals for the printheads that eject ink as the web passes by the printheads. Another environmental factor affecting the registration of images printed by different groups of printheads is web shrinkage. Web shrinkage is caused as the web is subjected to relatively high temperatures as the web moves along the relatively long path through the web printing system. The high temperatures drive moisture content from the web, which causes the web to shrink. If the physical dimensions of the web change after one group of printheads has formed an image in one color ink, but before another group of printheads has formed an image in another color of ink, then the registration of the two images is affected. The change may be sufficient to cause registration errors between ink patterns ejected by the different groups of printheads. The amount of shrinkage depends upon the heat to which the web is subjected, the speed of the web as it moves over heated components, the moisture content of the paper, and the type of paper, for example. Additionally, the amount of water in the web alters the elasticity of the web and the computations for web velocities with those changes. Addressing the effect of web changes and environmental conditions to reflex printing calculations during operation of a web printing system is important for image registration in web printing systems.